Title page for ETD etd-08232007-113000

Multidisciplinary analysis of a polymetamorphic terrane, western New England

Degree

PhD

Department

Geology

Advisory Committee

Advisor Name

Title

Tracy, Robert J.

Committee Chair

Bodnar, Robert J.

Committee Member

Ratcliffe, N. M.

Committee Member

Sinha, A. Krishna

Committee Member

Sutter, John F.

Committee Member

Keywords

Geology

Date of Defense

1990-04-05

Availability

restricted

Abstract

A combined petrologic, structural, and geochronologic approach has
been used to characterize a zone in southwestern New England affected by
superimposed metamorphisms, and the unroofing history of an area that
records only the youngest metamorphism. The area chosen for the study
of polymetamorphism, northwestern Connecticut and adjacent New York
and Massachusetts, records Taconian (Ordovician) peak metamorphic
effects in the west and Acadian (Devonian) peak effects in the east. In
between, a complicated zone of overlap contains predominantly Taconian
mineral assemblages that have been variably affected by Acadian processes.
40Ar/39Ar data presented here indicate that the timing of the Taconian
thermal maximum was approximately 445 Ma, and that of the Acadian was
approximately 390 Ma. Cooling ages suggest that the highest-grade
Taconian assemblages at the present erosion surface remained at elevated
metamorphic conditions between the Taconian and Acadian thermal
maxima.

Rim compositions of the highest-grade porphyroblasts in the
Taconian zone generally yield P-T estimates that are inconsistent for
subareas within a given thin section. With the onset of Acadian
metamorphic overprinting conditions of approximately 500°C, the rim
compositions of porphyroblasts yield P-T estimates that are much more
consistent, and vary by only a maximum of ±50° C and 1 kb. These results
are interpreted to indicate that the rims of preexisting Taconian
porphyroblasts reequilibrated pervasively at approximately 500°C and
above, whereas reequilibration was incomplete at lower Acadian overprinting temperatures. Garnet developed texturally and chemically
distinct rims which appear to reflect prograde Acadian metamorphism in
higher-grade parts of the Acadian metamorphic zone.

Previous studies of garnet zoning have emphasized volume and
surface diffusion as the primary means of changing mineral composition.
However, in this polymetamorphic zone changes in the composition of
preexisting garnet occurred by metasomatic dissolution and reprecipitation
along porphyroblast surfaces in the presence of a metamorphic fluid.
Diffusion seems significant for changing the garnet composition during the
initial overprint only as an agent to move material in an intergranular
fluid.

The pressures and temperatures of the Acadian metamorphism a consistent metamorphic field gradient of 16 bar/°C, increasing eastward.
In an area mainly affected by Acadian metamorphism, peak. metamorphic
temperatures were 8.2 kb and 575°C. Nonlinear unroofing rates from ~10
to less than 1 mm/yr following the high-pressure Acadian metamorphism
have been documented using combined petrologic, fluid inclusion,
thermochronometric, and thermal modeling techniques. The calculated
unroofing path is initially nearly isothermal and is followed by more
isobaric cooling. Comparison of the inferred path to those predicted by
thermal models suggests unroofing was characterized by initial rapid
upward en bloc velocity (~ 1 cm/yr) of brief duration, followed by much
slower unroofmg rates (≤ 0.3 mm/yr). This proposed unroofing history is
consistent with the Paleozoic sedimentary and igneous record of western
New England.